This invention relates to data processing apparatus and in particular to instruction fetching in such apparatus.
In program controlled data processing apparatus it is usual for program instructions to be held in a main store and to be fetched from the store for execution when required. The speed of operation of such apparatus is clearly restricted by the time required to fetch an instruction. It has also been proposed to provide, in association with the main store, a smaller but faster slave store which is arranged to hold a copy of information in the main store, which is in current use, thus permitting very fast access to that information. The provision of such a slave store can greatly reduce the time required to fetch an instruction, and can thus increase the speed of operation of the apparatus, especially where the program contains a loop of instructions which can be wholly contained within the slave store, since in that case the loop can be performed repeatedly without any access to the main store. However, the slave store is not so effective in the case where the program contains a many sequences of instructions without loops, since in that case a large number of instructions will have to be fetched from the main store.